1. Field of the Invention
The invention relates in general to transport refrigeration, and more specifically to air delivery systems for transport refrigeration units.
2. Description of the Prior Art
It is common in the transport refrigeration industry to have a dual speed prime mover, such as a Diesel engine, connected to drive a refrigerant compressor at a selected one of high and low speeds, such as 2200 RPM and 1400 RPM. The speed setting is responsive to the cooling or heating capacity demanded by a thermostat which compares the temperature of a served space with a temperature set point. If the condenser and evaporator blowers are directly driven by the engine, the air flow drops when the engine speed drops. This is desirable for the condenser blower, as the requirements on the condenser blower are reduced at the lower refrigeration capacity. This may or may not be desirable for the evaporator blower, depending upon the temperature set point. If the set point is set for unfrozen loads, such as produce, it is desirable to maintain a high air circulation rate in the served space at all times, regardless of compressor speed, in order to maintain a uniform temperature in the cargo and prevent "top freezing". If the set point is set for frozen loads, the temperature variation of the served space is not as critical, as long as all points of the cargo are well below the freezing point. Thus, reducing the air flow in the served space at the lower speed is beneficial, as the reduced power draw by the blower and the reduction of "fan heat" due to reduced compression and agitation of the air, translate into increased cooling capacity, adding as much as 10 to 15% capacity, and reduced fuel consumption as well.
Thus, while some systems include means for automatically maintaining the evaporator air flow constant regardless of compressor speed, this is not always advantageous, and can be a disadvantage.
In my co-pending application Ser. No. 270,861 filed Nov. 14, 1988, entitled "Air Delivery System For A Transport Refrigeration Unit", now U.S. Pat. No. 4,878,360, there is disclosed an adjustable speed arrangement which includes a jack shaft driven by the prime mover. The jack shaft has a fixed pulley which drives the condenser blower at a speed directly proportional to the speed of the prime mover, and a variable pulley. The variable pulley is linked by a belt which also links a pulley which drives the evaporator blower, and an idler pulley. The position of the idler pulley is controlled by a linear actuator to tension the belt and select the pitch diameter of the variable pulley which will provide the desired evaporator air flow for the temperature set point, at the current prime mover speed. While this arrangement works well, it would be desirable, and it is the object of the present invention, to improve upon the arrangement of my co-pending application, providing a simpler and lower maintenance air delivery arrangement for a transport refrigeration system in which the speed of the condenser blower is directly proportional to the speed of the prime mover, while the speed of the evaporator blower is selectable.